Oil accumulated within a subterranean formation can be recovered, or produced, through wells from the formation using the energy that is inherent within the formation. However, producing operations deplete the natural energy relatively rapidly. Thus, a large amount of the oil is left in the subterranean formation if only the natural energy is used to produce the oil. This production by depletion of the natural energy is often referred to as primary production. Where natural formation energy is inadequate or has become depleted, supplemental operations, often referred to as secondary or tertiary recovery operations, are used to improve the extent of recovery of the oil. In one of the most successful most widely used supplemental recovery operations, a fluid is injected through an injection means, comprising one or more injection wells. The fluid is passed into the formation, displacing oil within the formation and moving it through the formation. The oil is produced from production means, comprising one or more production wells, as the injected fluid passes from the injection means toward the production means. In a particular recovery operation of this sort, water is employed as the injected fluid, and the operation is referred to as water flooding. The injected water is referred to as flooding water as distinquished from the in-situ, or connate, water.
While conventional water flooding is effective in obtaining additional oil from oil-containing subterranean formations, it has a number of shortcomings. Foremost among the shortcomings is a tendency of the flooding water to "finger" through an oil-containing formation and to bypass substantial portions thereof. By fingering is meant the developing of unstable bulges or stringers which advance toward the production means more rapidly than the remainder of the flooding water. Furthermore, the water does not normally displace as much oil in the portion of the formation which it contacts as it is potentially capable of doing.
It has been established that water floods perform less satisfactorily with viscous oils than with relatively nonviscous oils. The fingering and bypassing tendencies of the water are more or less directly related to the ratio of the viscosity of the oil to the viscosity of the flooding water. The viscosities of different oils vary from as low as 1 or 2 centipoises to as high as 10,000 centipoises or higher. Water has a viscosity of about 1 centipoise.
Past suggestions for increasing the viscosity of flooding water have included incorporating a water-soluble thickening agent in the water. Additives that have been suggested for this purpose include a wide variety of naturally occurring gums, sugars and polymers. The bacterially produced heteropolysaccharides have been especially interesting as an additive for thickening flooding water.
Typical of the prior art approaches are those disclosed in U.S. Pat. No. 3,352,358; "Water Flood Employing Viscous Aqueous Solutions"; issued Nov. 14, 1967 and disclosing the injection of an aqueous solution of polyvinylalcoholsulfate as a viscous fluid for displacing the oil and for improving the mobility ratio. Similarly, U.S. Pat. No. 3,372,749; "Water Flood Process Employing Thickened Water," issued Mar. 12, 1968, disclosed the process for injecting a poly(glucosylglucan), for the same purpose.
While these additives are effective to an extent in increasing the viscosity of the flooding water, they are also characterized by one or more serious disadvantages. Some of the additives are effective only in restricted ranges of pH. Many of the additives form insoluble precipitates with divalent ions, such as calcium ions, either in the flooding water or in the in-situ fluids. A disadvantage suffered by many of the aqueous solutions of these additives is the adverse reduction of viscosity of the aqueous solution when contacted with brines, such as solutions containing sodium chloride or calcium chloride.
One particular disadvantage of these procedures of injecting aqueous solutions of polymers is the diminshed production during the injection of the polymer solution because of the decreased injectivity in the region immediately about the injection means.
Expressed otherwise, the method of injecting the fluid should have the features not heretofore provided:
(1) The method should achieve relatively high injectivity in the region immediately adjacent the injection well, or injection means; but should achieve increased viscosity out in the formation to produce a more nearly equal mobility ratio between the fluid and the oil that the fluid is displacing.
(2) It is particularly desirable that the injection of the fluid does not effect a diminished production of oil during the time of injection of the polymer solution.